1. Field of the Invention
This invention relates to low pressure thermoforming of polyalkylene terephthalate (PAT) resins.
2. Prior Art
Three examples of art disclosing thermoforming of polyalkylene terephthalate resin sheets are two to Siggel et al. U.S. Pat. No. 3,429,854 (1969) and U.S. Pat. No. 3,496,143 (1970), and one to McTaggart U.S. Pat. No. 3,960,807 (1976).
In Siggel et al. (U.S. Pat. No. 3,429,854), the thermoforming process disclosed consisted of: (1) heating with the I.R. heating system of an Illig Type U60 a polyethylene terephthalate sheet of 0.1 to 6 mm in thickness, having a percent crystallinity between 0% and 25% and a solution viscosity between 1.4-1.7 as measured in 1% meta cresol at 77.degree. F., to a temperature in the range 167.degree. F. to 356.degree. F.; (2) shaping by means of atmospheric pressure on a mold surface (no temperature of the mold surface indicated), and (3) cooling immediately after shaping to a temperature below 158.degree. F., whereby a thermoformed transparent sheet can be formed having a draw ratio of maximum depth to maximum width as high as 2.5:1. A funnel shaped mold wherein the maximum diameter equals 150 mm and the minimum diameter at a distance of 150 mm equals 20 mm was used along with some physical prestretching of the sheet prior to shaping on the mold in those cases where a deep-draw ratio in excess of about 1:1 was sought.
In Siggel et al. (U.S. Pat. No. 3,496,143), the thermoforming process disclosed consisted of: (1) heating with the I.R. heating system of an Illig Type U60 a polyethylene terephthalate sheet of 0.1 to 6 mm in thickness, having a percent crystallinity prior to heating between 5%-25% and a solution viscosity or relative viscosity as measured in a 1% meta cresol solution at 77.degree. F. in the range 1.7 to 2.0, to a temperature between 185.degree. F. and 392.degree. F., (2) immediately shaping by means of atmospheric pressure on a mold surface (no temperature of the mold surface is indicated) while the I.R. heating system is turned off, and (3) further heating said shaped sheet with said heating system so as to maintain a temperature in the range 284.degree. F. to 428.degree. F. until a percent crystallinity of at least 25% is obtained. It is not made clear by the reference whether it is necessary or not to turn off the heating system during the shaping step or what the temperature of the mold surface is during the shaping step.
In McTaggart (U.S. Pat. No. 3,960,807),, the thermoforming process disclosed consisted of:
(1) heating an extruded 20 mil sheet containing:
(a) polyethylene terephthalate having an inherent viscosity, as determined in a dilute trifluoroacetic acid solution at 86.degree. F., of at least 0.75, a glass transition temperature of at least 122.degree. F., and a melting point of at least about 302.degree. F.; PA1 (b) a crack stopping agent; and PA1 (c) a nucleating agent; to a temperature on the order of 250.degree. F., PA1 (a) contacting said sheet with a mold which is at a temperature in the range of about 240.degree.-350.degree. F. and preferably in the range of about 270.degree.-310.degree. F. wherein said mold is preferably at a higher temperature than said sheet prior to contact therebetween; PA1 (b) applying forces comprising a differential in applied pressure to said sheet so as to make a thermoformed sheet which conforms to said mold; PA1 (c) allowing said thermoformed sheet to remain in contact with said mold until an average crystallinity of at least 20% is achieved; PA1 (d) removing said thermoformed sheet from said mold; and PA1 (e) allowing said thermoformed sheet to cool. PA1 (1) forming said sheet so that it is substantially unoriented, has a thermally induced average crystallinity in the range of about 0-10%, and has a half-time for crystallization at 410.degree. F. below about 5 minutes and preferably below about 3 minutes and above about 0.5 minutes, PA1 (2) heating said sheet to an average temperature in the range of about 210.degree.-300.degree. F. and preferably in the range of about 225.degree.-270.degree. F. at such a rate (generally for example on the order of 10.degree.-25.degree. F./sec.) that by the time that said sheet has reached said temperature said sheet has an average crystallinity in the range of about 0% to 18%, and preferably about 5% to 15%, PA1 (3) thermoforming said sheet by: PA1 (4) allowing said thermoformed sheet to remain in contact with said mold until an average crystallinity of at least 20% is achieved; PA1 (5) removing said thermoformed sheet from said mold; PA1 (6) allowing said thermoformed sheet to cool; and PA1 (7) trimming said thermoformed sheet. PA1 (a) contacting said sheet with a deep-draw mold which is at a temperature in the range of about 240.degree.-350.degree. F. and preferably in the range of about 270.degree.-310.degree. F. wherein said mold is preferably at a higher temperature than said sheet prior to contact therebetween; PA1 (b) applying forces comprising a differential in applied pressure to said sheet so as to make said sheet conform to said mold, whereby a thermoformed sheet having a draw ratio in excess of about three can be achieved; PA1 (c) allowing said thermoformed sheet to remain in contact with said mold until an average crystallinity of at least about 20% is achieved; and PA1 (d) removing said thermoformed sheet from said mold.
(2) shaping said sheet to a mold (mold temperature during shaping is not given); and
(3) raising the temperature of the mold to a temperature in the range of 275.degree. F. to 374.degree. F. and allowing the shaped sheet to remain in contact with the mold for 20 seconds to 3 minutes.
The average crystallinity preferred at various points in the process are not given. In one example, an initially amorphous sheet after shaping to a mold surface and prior to heat annealing is expressly disclosed to still be amorphous.
It is to be noted that average crystallinity of a polyalkylene terephthalate (PAT) sheet is given as a percent by weight of crystalline material as based upon the total weight of said sheet and is throughout the Specification and Claims, unless otherwise specifically indicated, thermally induced. Shear induced crystallinity in PAT, even if followed by thermal annealing, will have a different crystalline morphology resulting in generally lower melting and softening temperatures than PAT having only thermally induced crystallinity.
It is known that if sufficiently large pressures on the order of 10-50 atmospheres are used shaped articles can be formed from sheets of PAT having a thermally induced average crystallinity in the range of about 25-30%. In order to achieve said sufficiently large pressures, large and very expensive machinery must be employed.
It is further known that, in general, if a PAT sheet has too much thermally induced average crystallinity, i.e. above about 50%, it becomes very brittle and loses many of its desirable properties. The final overall crystallinity of a PAT sheet is known to depend upon the concentration and particle size of any crystallization promoter therein. Rate of crystallization for example tends to increase both with a decrease in particle size of said promoter and, at a concentration up to about 1%, with an increase in concentration. Final average crystallinity after annealing, however, depends only on concentration of a particular crystallization promoter. A concentration of such promoters above about 1% generally results after annealing in a PET sheet having a thermally induced average crystallinity above 60%.